feat: per-phase timings of the frame path in the log

One log line per phase every PHASE_LOG_EVERY samples (default 100, 0 = off):
mean and max of wait_frames, align, convert, each JPEG/PNG encoding,
colormap, depth_stats, base64, json and ws_send. Meant for the measurement
on PC2 via `docker logs` (optimize-streams R01): where a capture spends its
~130 ms of CPU is not known yet.

capture() moves its numpy conversion into _frame_from() unchanged, only so
the block can be timed; _depth_colormap() likewise wraps _jet_colormap().

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/backend/services/camera.py

@@ -30,6 +30,8 @@ import cv2
 import numpy as np
 import pyrealsense2 as rs
 
+from src.backend.services.phase_timing import timer
+
 logger = logging.getLogger(__name__)
 
 
@@ -376,19 +378,28 @@ class RealSenseD435:
         if not self._open:
             return None
         try:
-            frames = self._pipeline.wait_for_frames(timeout_ms)
+            with timer.phase("wait_frames"):
+                frames = self._pipeline.wait_for_frames(timeout_ms)
         except Exception as e:
             logger.warning(f"wait_for_frames failed: {e}")
             return None
 
         if self._align is not None:
-            frames = self._align.process(frames)
+            with timer.phase("align"):
+                frames = self._align.process(frames)
 
         color_frame = frames.get_color_frame()
         depth_frame = frames.get_depth_frame()
         if not color_frame or not depth_frame:
             return None
 
+        with timer.phase("convert"):
+            frame = self._frame_from(color_frame, depth_frame)
+        self._last_frame = frame
+        return frame
+
+    def _frame_from(self, color_frame, depth_frame) -> CameraFrame:
+        """numpy BGR + depth in mm from the SDK frames (YUYV decode, USB 2 resize, depth scale)."""
         if self._color_format == rs.format.yuyv:
             # SDK exposes YUYV as a 1-channel view of the packed buffer;
             # rebuild it as (H, W, 2) uint8 so cv2 can decode it to BGR.
@@ -417,9 +428,7 @@ class RealSenseD435:
         else:
             depth_mm = (depth_raw.astype(np.float32) * self._depth_scale * 1000.0).astype(np.uint16)
 
-        frame = CameraFrame(rgb=rgb, depth=depth_mm, timestamp=time.time())
-        self._last_frame = frame
-        return frame
+        return CameraFrame(rgb=rgb, depth=depth_mm, timestamp=time.time())
 
     def _cached_or_capture(self) -> Optional[CameraFrame]:
         if (